The present invention relates generally to a conduit system for conveying a fluid therethrough, and more particularly to a system and method of controlling diffusion of the fluid through the conduit, such as may be used in a hardcopy printing mechanism for conveying ink from a reservoir to a printhead.
Whenever there is a need for transporting a fluid, such as a liquid, suspended particulate, or gaseous material, through a conduit, such as a channel or tube, there are several factors which are of concern. One factor involves the amount of fluid which will be lost due to diffusion through the conduit walls. Another factor in a flexible conduit system, which has one end of the conduit moving relative to the other end, involves the amount of space required for the conduit to flex during movement. An additional factor concerns how tightly the conduit may be bent during this movement before its walls collapse. This maximum degree of bending before the conduit walls collapse is a characteristic of conduit referred to as the "bend radius."
For example, FIG. 9 shows a perspective view of a sectioned prior art flexible tube A, having a round radial cross sectional profile. The tube A has concentric inside and outside diameters, D.sub.i and D.sub.o, respectively. The tube A is shown as being bent to its minimum bend radius R' about a center point C'. For round tubing, the bend radius R' is dependent upon the relationship between the inside and outside diameters D.sub.i, D.sub.o.
Thus, there are a variety of competing concerns to balance when using the prior art round tubing A (FIG. 9) to convey a fluid between two locations, particularly when one location is moving relative to the other. One example of a need for a moving flexible conduit is an "off-axis" inkjet printing mechanism, such as that used in plotters, facsimile machines and inkjet printers, which have a remote reservoir for storing a colorant, referred to generally herein as "ink." Inkjet printing mechanisms typically have a printhead which is propelled from side to side across a print medium, such as paper, with the printhead being controlled to selectively deposit ink in a desired pattern on the page. While some inkjet print mechanisms carry an ink cartridge along with the printhead back and forth across the sheet, an "off-axis" system propels only the printhead across the printing region, with the ink supply being stored in a stationary reservoir. Typically, a flexible conduit is used to convey the ink from the reservoir to the printhead.
These off-axis printing mechanisms may be in continual use, but often they are used only intermittently. While sitting during periods of inactivity, the ink remaining in the tubing from the last use is prone to diffusion through the tube Walls. Worse yet, only certain components of the ink may diffuse through the tube walls, leaving a composition in the tubing which is far different than the original ink formulation.
For instance, alcohol-based inks are prone to alcohol diffusion through the tube walls. While lack of the alcohol component will not necessarily cause these inks to dry out and clog the tubing, the ink which eventually reaches the print medium often dries slower than optimum, and it is often subject to color bleeding. To address this problem, some inkjet printers purge the entire ink conduit each time the printer is activated. This solution can be particularly wasteful for some light-duty users, since nearly an entire month's supply of ink may be stored in the tubing.
One conventional manner of addressing the diffusion problem is to increase the wall thickness of the conduit. Unfortunately, as the wall thickness increases, the bend radius also increases. A larger bend radius requires additional space, so often the overall size of the device housing the conduit must be larger, which increases material and shipping costs. Furthermore, a physically larger unit impedes the goal of providing more compact equipment with a smaller "footprint," i.e., the physical area occupied by the equipment in the workplace or home environment.
Another manner of minimizing diffusion is to add diffusion inhibitors to the tubing material. These anti-diffusion additives tighten the molecular bonds of the conduit material, which unfortunately has the side effect of increasing the stiffness of the tubing. Stiffer tubing requires more force to bend. To handle these additional forces, portions of the equipment which house the tubing must often be upgraded. For instance, in an inkjet printing mechanism, a stiffer tube requires heavier printhead carriage bearings, a more powerful motor to propel the carriage across a page, and integrated circuit chips with higher voltage ratings. A stiffer tube also requires a larger bend radius R'. A larger bend radius, whether caused by thickening the tube walls, or adding diffusion inhibitors to the tube material, requires the conduit be housed in a physically larger device.
Thus, the earlier flexible conduit systems have inadequately addressed the competing needs of minimizing diffusion and minimizing the bend radius, as illustrated above with respect to an inkjet printer. The solutions used in the past have often minimized the diffusion rate at the expense of increasing the bend radius, which increases the overall size of the resulting equipment. Larger equipment is often heavier and more costly to manufacture and ship, as well as being undesirable to some customers who prefer more compact equipment with a smaller footprint.